Hollow cathodes are used to emit electrons in a variety of industrial applications. As described in a chapter by Delcroix, et al., in Vol. 35 of Advances in Electronics and Electron Physics (L. Marton, ed.), Academic Press, New York (1974), beginning on page 87, there are both high and low pressure regimes for hollow-cathode operation. In the high-pressure regime, the background pressure (the pressure in the region surrounding the hollow cathode) approaches or exceeds 1 Torr (130 Pascals) and no internal flow of ionizable working gas is required for operation. In the low-pressure regime with a background pressure below 0.1 Torr, an internal flow of ionizable working gas is required for efficient operation. It is for operation in the low-pressure regime below 0.1 Torr, and usually below 0.01 Torr, that the present invention is intended.
An important industrial application of low-pressure hollow cathodes is for electron emission in ion sources. These ion sources are of both gridded and gridless types. The ions generated in gridded ion sources are accelerated electrostatically by the electric field between the grids. Gridded ion sources are described in an article by Kaufman, et al., in the AIAA Journal, Vol. 20 (1982), beginning on page 745. The particular sources described in this article use a direct-current discharge to generate ions. It is also possible to use electrostatic ion acceleration with a radio-frequency discharge, in which case the only electron emitting requirement would be for a neutralizer cathode.
In gridless ion sources the ions are accelerated by the electric field generated by an electron current interacting with a substantial magnetic field in the discharge region, i.e., a magnetic field with sufficient strength to make the electron-cyclotron radius much smaller than the length of the discharge region to be crossed by the electrons. The closed-drift ion source is one type of gridless ion source and is described by Zhurin, et al., in an article in Plasma Sources Science & Technology, Vol. 8, beginning on page R1, while the end-Hall ion source is another type of gridless ion source and is described in U.S. Pat. No. 4,862,032—Kaufman, et al.
There are different types of low-pressure hollow cathodes. The simplest is a refractory-metal tube, usually of tantalum. This type is described in the review by Delcroix, et al., in the aforesaid chapter in Vol. 35 of Advances in Electronics and Electron Physics. For hollow cathodes of the sizes, electron emissions, and gas flows of most interest herein, the lifetime of these simple cathodes is limited to a few tens of hours.
Another type of hollow cathode has been developed for electric thrusters used in space propulsion and is described in a chapter by Kaufman in Vol. 36 of Advances in Electronics and Electron Physics (L. Marton, ed.), beginning on p. 265. The distinguishing feature of this type is an emissive insert that emits electrons at a lower temperature than does the plain metal-tube of the first type. The major advantage of this type is the long lifetime that is possible, of the order of 10,000 hours. The major disadvantage is the sensitivity of the supplemental emissive material to contamination. The emissive insert incorporates the supplemental emissive material that starts out as a carbonate (most often barium carbonate) and becomes an oxide when it is initially heated, or conditioned, for operation. If it is exposed to air after operation, the oxide combines with the water vapor in the air to become a hydroxide, which is much less effective as an emission material. Repeated exposure to air is not a problem in the space electric-propulsion application for which these cathodes were originally designed, but is much more serious in industrial applications.
A hollow cathode for industrial applications should have an operating lifetime of at least several hundred hours and be insensitive to repeated exposures to atmosphere between periods of operation. Shorter lifetimes than several hundred hours would be a problem because the time between maintenance in many industrial applications would then be limited by the cathode lifetime. While longer lifetimes might be of interest for industrial hollow cathodes, the time between maintenance would probably still be limited by other system components. In other words, the cost of a longer-lifetime hollow cathode, together with any special care and handling required, would have to be balanced against the replacement cost of a new hollow cathode of a simpler type.
The refractory metal tube of Delcroix, et al., in the aforesaid chapter in Vol. 35 of Advances in Electronics and Electron Physics is simple and, made of a metal such as tantalum, can stand repeated exposures to atmosphere between periods of operation. Its major shortcoming is a short lifetime. The space-propulsion hollow cathode described by Kaufman in the aforesaid chapter in Vol. 36 of Advances in Electronics and Electron Physics has a more than adequate lifetime, but is more complicated and more expensive, both to make and to use. For operation with frequent exposures to atmosphere, it is best to keep an inert gas flowing through such a cathode during atmospheric exposures to prevent degradation of the low-work-function, low-temperature emissive material. Even then, contamination from various gases used in the industrial application will probably limit the lifetime to far less than would be obtained in a space environment.
What might be called a compromise of the two types of hollow cathodes has been used in industrial applications. In this type, an emissive insert is used, but this insert consists only of tantalum foil. The lifetime is not as long without a low-work-function emissive material such as barium carbonate, but the tantalum-foil insert is less sensitive to atmospheric exposure than an insert that depends on the addition of an emissive material. Even with the reduced sensitivity to atmospheric exposure, a common mode of failure is oxidation of the tantalum foil and having it break into flakes, eventually clogging the flow passage through the tantalum-foil insert.
Another example of possible hollow-cathode configurations is U.S. Pat. No. 5,587,093. There is described a hollow cathode with multiple radiation shields surrounding a tube through which the working gas is introduced. However, there are intervening support structures between both the tube and the inner radiation shield and between the inner and outer radiation shields. These support structures permit a large fraction of the escaping heat to be conducted by the support structures around the ends of the radiation shields, thereby degrading the effectiveness of the radiation shields. Aston also uses an electrically heated emissive insert, a component not used in the present invention.